As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which exhibit high energy density and voltage and have long cycle lifespan and a low self-discharge rate, are commercially available and widely used.
However, conventional lithium secondary batteries may catch fire or explode when exposed to high temperature. In addition, when a large amount of current flows in a short time due to overcharging, external short circuit, nail penetration, crushing, or the like, the batteries are heated due to IR heat and, as such, the batteries may catch fire or explode.
That is, as the temperature of the battery is increased, reaction between the electrolyte and the electrodes is accelerated. As a result, reaction heat is generated and, thus, the temperature of the battery is further increased, which accelerates the reaction between the electrolyte and the electrodes. This feedback loop causes a thermal runaway phenomenon in which the temperature of the battery is sharply increased. When the temperature of the battery is increased to a predetermined temperature level, the battery may catch fire. In addition, as a result of the reaction between the electrolyte and the electrodes, gas is generated and, thus, the internal pressure of the battery is increased. When the internal pressure of the battery is increased to a predetermined pressure level, the lithium secondary batteries may explode.
Lithium metal oxides most widely used as a positive electrode of lithium secondary batteries are generally formed by reacting lithium carbonate and carbonate. When a stoichiometric amount of the lithium carbonate is increased, a residue of the lithium carbonate is decomposed and, thus, a variety of gases such as carbon dioxide, carbon monoxide, hydrogen, and the like are generated.
Meanwhile, an electrolyte reacts with impurities and lithium ions on a negative electrode surface of batteries even during initial charging and, thus, the electrolyte is also decomposed during formation of a solid electrolyte film, followed by generation of gases.
Gases such as carbon dioxide and the like generated within a battery may be reversibly returned to original materials during charging according to conditions. However, in most cases, the generated gasses remain within a battery in a gaseous state, thereby increasing internal pressure of the battery and causing swelling of the battery. The thickness of the swelled battery is increased and, as such, the thickened battery may not be easily installed in electric and electronic devices. Alternatively, due to a bulged appearance of the battery, the battery is judged to be defective and, thus, commercial value thereof is lost.
Therefore, one particular essentially considered to develop a lithium secondary battery is to secure stability by preventing or removing generation of inner gases inducing ignition/explosion and swelling at high temperature.
In efforts to secure such stability, there are conventionally a method of installing a device outside a cell and a method of using a particular material within a cell. However, such methods do not perform a normal protection role in cases requiring fast response time such as internal short circuit, needle penetration, local damage, and the like and performance of the battery may also be deteriorated due to addition of the material.
Therefore, there is an urgent need for development of new technology to prevent ignition/explosion due to gas generation without deterioration of overall battery performance.